The present invention relates to thin film resistors. More particularly, the present invention relates to integration of a plurality of such thin film resistors.
Thin film resistors are often used in integrated circuits to implement the desired functionality of the circuit, including biasing of active devices, serving as voltage dividers, and assisting in impedance matching. These thin film resistors may be formed by deposition of resistive material on a dielectric layer, and subsequently patterned to desired size and shape. Often, a thin film resistor is subjected to a heat treatment process (i.e. annealing) to improve its stability and to bring the resistance to a desired value.
Generally, after all the thin film resistors and other components of an integrated circuit are formed, a dielectric layer is deposited to insulate the resistors and other components from the interconnect wiring. This dielectric layer may be subjected to planarization by chemical-mechanical polishing (CMP) if reduced topography is desired before forming the interconnect wiring. Once the dielectric layer is formed, contacts are made through the layer to make electrical connections to thin film resistors and other components of the integrated circuit.
Problems with simultaneously forming contacts to thin film resistors and active components built into the silicon substrate include a thickness difference of the dielectric above the thin film resistors and active components. Moreover, long etch times required to open the deepest contact (to the substrate) may result in a large over-etch in shallower contacts (to thin film resistors). This over-etch may etch through or sputter away the thin film material. Accordingly, the thin film resistor contacts may be formed by developing and patterning etch-stops to alleviate the thickness variations. However, the formation of etch-stops, in addition to the contacts, increases the number of masks required. Furthermore, the formation of multiple thin film resistors, along with the associated contacts and etch-stops, may require a large number of masking operations.